Heat a pan or skillet over medium heat and melt the butter. Add the diced onions and sauté for about 2 minutes. Add the grated garlic and cook for another 2 minutes, stirring occasionally.
Add salt, black pepper, red chili flakes, oregano, and mixed herbs. Stir well to combine, then add the chopped coriander. Cook for another 2 minutes, stirring occasionally, until the onions turn golden brown and soften.
Pour in the milk and stir continuously until the mixture comes to a gentle boil.
Add the cheese slices (or grated cheddar) and stir until the cheese melts completely and the mixture turns smooth and creamy. This should take about 1 minute.
Transfer the caramelized onion mixture to a medium bowl and let it cool for about 30 seconds.
Toast the bread slices until crisp and golden. Spread a generous layer of the creamy caramelized onion spread over each slice. 7. Finish with a sprinkle of freshly chopped coriander and red chili flakes.